Relationship between critical current and flux-flow resistivity in the mixed state of Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$

Abstract

We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed-state of unconventional superconducting alloys Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($0.044\le x\le 0.100$) through current-voltage measurements. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point ($j_0,E_0$) and that there is a simple relationship between the critical current density $j_c$ and flux-flow resistivity ${\rho }_{\mathrm{ff}}$: ${\rho }_{\mathrm{ff}}/{\rho }_{\mathrm{n}}=(1-j_c/j_0{)}^{-1}$, where ${\rho }_{\mathrm{n}}=E_0/j_0$ is the normal-state resistivity just above the superconducting transition. In addition, $E_0$ is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity ${\rho }_{\mathrm{ff}}$: it increases with decreasing magnetic field. In contrast, $E_0$ is negative for the conventional superconductor Nb since, as expected, ${\rho }_{\mathrm{ff}}$ decreases with decreasing magnetic field. Furthermore, in the under-doped and over-doped single crystals of Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$, the parameter $E_0$ remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping ($0.060\le x\le 0.072$). This result points to the co-existence of superconductivity with some other phase around optimal doping.

arXiv comment6 pages, 6 figures

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